Sulfolane, C4H8O2S is also known as tetrahydrothiophene-1,1-dioxide and tetramethylene sulfone. Sulfolane is used as an extraction and a reaction solvent. It is used to separate aromatic hydrocarbons (benzene, toluene and xylenes) from non-aromatics, such as paraffins and naphthenes. It is used to separate n-propyl alcohol and sec-butyl alcohol. It is used to purify natural gas streams and for fractionation of fatty acids into saturated and unsaturated components.
A sulfolane unit is usually incorporated within an aromatics complex to recover high-purity benzene and toluene products from reformate and pyrolysis gasoline. In a modern aromatics complex, the sulfolane unit is located downstream of the reformate splitter column.
In sulfolane extraction column, aromatics are dissolved in sulfolane and this mixture is the extract phase (bottom), while the remaining non-aromatics counter-currently contacted and efficiency of stage can be improved by mixing with rotating baffles such as rotating disk column, RDC.
Sulfolane is recovered from raffinate first in a coalescer and then in a water wash column. The final raffinate is pumped to Naphtha Blending as gasoline components.
The extract is stripped and enriched in aromatics after recontacting with additional sulfolane. The solvent is recovered under vacuum distillation from the extract; while the extract is separated from water by cooling and clay treated to eliminate olefins and diolefins, and sent to Aromatics Fractionation Unit to separate benzene, toluene.
In the sulfolane extraction system, there are often severe corrosion problems. The locations most often corroded by contaminated sulfolane include the rich solvent line to the stripper, extractive stripper column, reboilers, transfer lines and solvent recovery column. As the corrosion rate increases, pinholes can develop, leading to safety and hazard concerns.
The root cause of the corrosion is mostly the accumulation of acidic material in the circulating solvent. The contaminated sulfolane turns from colorless to black, becomes more viscous and gooey, acidic (to a pH level as low as 4) and corrosive. In general the following four items are seen as the main causes for corrosion/erosion problems in the sulfolane extraction unit.                Oxygen in the plant (ingress or via incoming flows)        Chlorine in circulating solvent        Accumulation of degradation and corrosion products in the plant        High temperature in reboilers        
The degradation and corrosion products are gooey, gummy, tacky, smelly, and difficult and unpleasant to handle. Over the years, degradation and corrosion products will have accumulated in the plant. These degradation and corrosion products will deposit in the system to plug the filters, carbon adsorption bed to increase the pressure drop and most importantly to reduce the heat transfer coefficient of the heat exchangers, leading to poor operation efficiency. In addition, the dirt accumulated in the plant is usually corrosive material and/or act as a kind of catalyst to make more degradation products. An expedient way to control corrosion is to neutralize the acid by addition of monoethanol amine (MEA), leading to additional contamination of the sulfolane solvent. The experience from the operation indicates that once in a while a very thorough cleaning of the plant is required. In a commercial sulfolane extraction unit, about 10-15 m3 of dirt was taken out of the unit. It is indeed, a hard job to clean the sulfolane extraction unit thoroughly.
There are many ways to keep the sulfolane in the circulation unit clean, including:                1) A filter bed or cartridge is placed in the lean sulfolane transfer line to catch the degradation and corrosion products. However, the filter or cartridge can be overcome by the large quantity of the degradation and corrosion products in short time. Most of the commercial units are equipped with such filters but the filters are mostly bypassed because of operational difficulties.        2) Twin adsorption towers filled with adsorbent such as activated carbon to remove the degradation and corrosion products. Commercial experience indicates that such adsorption tower is impractical because it can be plugged up quickly and it is difficult to clean and change the adsorbent.        3) Reclamation of sulfolane: A small split stream of sulfolane laden with the degradation and corrosion products is withdrawn from the circulation system and charged into a vacuum distillation unit. The overhead is clean sulfolane for recycling to the circulation system, while the bottom is the degradation and corrosion products, which are disposed as waste. The operation is inefficient because only a small fraction of the solvent in the system is reclaimed. In addition, there is significant amount of hazardous waste generated for disposal.        4) Use of cationic and/anionic resins to remove the acidic contaminants, including sulfonic acid, and small amount of carboxylic acid and hydrochloric acid. U.S. Pat. No. 5,053,137 by Lal and Bhat taught purification or regeneration of contaminated or spent process sulfolanes by passing a split stream of it through two columns in series. The first column is filled with cationic exchanger while the second column is filled with anionic exchanger. Liu and Li showed that the acid in the degradation and corrosion products could be effectively ion exchanged with cationic resin [Liu lingchi and Li Dong-sheng, Petrochemical Design, 2002, 19(2) 31-33], leading to cleaner sulfolane solvent. The acid laden cationic resin is regenerated with NaOH solution. In the process, only a split stream is ion exchanged, so that the efficacy in improving sulfolane quality is limited. The process is messy and produces a great quantity of hazardous waste for disposal.        
To control the corrosion rate, the acid can be neutralized by use of amines, such as monoethanol amine (MEA). However, the degradation and corrosion products remain in the circulation system to foul up the heat exchangers and to cause plugging and increase in pressure drop. In addition, the accumulated degradation and corrosion products catalyze the degradation of sulfolane due to autocatalysis and accelerate the corrosion and accumulation of degradation and corrosion products leading to a vicious cycle. Thus, a significant amount of purge is required to keep the quality of sulfolane at an acceptable level.
The prior art on controlling sulfolane corrosion and rejuvenating sulfolane in the circulation loop suffers from a number of disadvantages including:                1) High corrosion rate of the system because the accumulated degradation and corrosion products catalyze the degradation of sulfolane and accelerate the corrosion and accumulation of degradation and corrosion products, leading to a vicious cycle.        2) Accelerated rate of accumulation of the degradation and corrosion products due to their autocatalysis.        3) Tedious, dangerous and expensive operation in replacing the filters and/or adsorbents.        4) Decreased operation capacity and efficiency when the quality of sulfolane is not maintained at or higher than the desired level.        5) Inefficient in rejuvenation of sulfolane solvent due to split stream operation.        6) High operation and maintenance costs.        
Clearly, there is a need in the industry for an efficient, safe and economic method to rejuvenate the contaminated sulfolane to operate the aromatic extraction system safely, efficiently with high capacity, and low operation and maintenance costs.